Universal Mobile Telecommunications System (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in Wideband Code Division Multiple Access (WCDMA) based on European systems, Global System for Mobile communications (GSM) and General Packet Radio Services (GPRS). The Long Term Evolution (LTE) of UMTS is under discussion by the 3rd Generation Partnership Project (3GPP) which standardized UMTS. The objective of the LTE work is to develop a framework for the evolution of the 3GPP radio-access technology towards a high-data-rate, low-latency and packet-optimized radio-access technology. So the focus is on supporting services provided from the packet switched (PS)-domain.
A key goal of the 3GPP LTE technology is to enable high-speed packet communications at or above about 100 Mbps. Many schemes have been proposed for the LTE objective including simplifying the radio network structure and optimizing the radio protocol for radio channels. The working assumption that the downlink will use Orthogonal Frequency Division Multiplexing (OFDM) and the uplink will use Single Carrier-Frequency Division Multiple Access (SC-FDMA). Supported downlink data-modulation schemes include QPSK, 16QAM, and 64QAM. Possible uplink data-modulation schemes are (π/2-shift) BPSK, QPSK, 8PSK, and 16QAM. In addition, Multiple Input Multiple Output (MIMO) will likely be used with multiple antennas at the mobile and at the cell site.
Another important LTE parameter is short access latency by the user equipment (UE) as soon as data is to be transmitted from or received by the UE. Lower latency is beneficial, not only for traditional end-to-end calls, but also for other services like web-browsing, email, etc. For example, a mobile terminal user might be web browsing. After reading a web page, the use may want to retrieve another related web page, email a friend about the contents of that page, etc. It would be desirable for the user to be able to perform those operations immediately without having to wait for a connection to be set up. This will give the user the same type of experience as expected using a fixed wire broadband service.
FIG. 1 illustrates the radio interface protocols needed to set up, reconfigure, and release radio bearer services in UTRAN. The protocol is divided into a user plane where the user data is handled, and a control plane where control signaling is handled by the radio resource control (RRC). RRC messages carry all parameters required to set up, modify, and release layer 2 and layer 1 entities as well as the messages relating to the mobility of the UEs including measurements, handovers, cell updates, etc. In UTRAN, a powered-on UE assumes either an idle mode or a connected mode. The connected mode in UTRAN includes four radio resource control (RRC) states: cell DCH state, cell FACH state, cell PCH state, and URA PCH state. For purpose of this application, the term “connected” means that a communication connection has been established between a UE and a radio access network (RAN).
In LTE, there will be just two RRC states: RRC idle state and RRC connected state. If a UE is in an RRC idle state, the UE has selected a mobile network to contact and a cell of that selected mobile network to provide services. After camping on a cell in the idle state, the UE can receive system information and cell broadcast messages. The E-UTRAN itself does not know in which cell the UE is located and only knows the UE's location in the context of a much larger tracking or paging area. Moreover, a UE in the idle state has not been currently authenticated to the network and does not have a current local IP address. Because it takes some time for the UTRAN to locate the idle UE, authenticate it, assign a local IP address, and perform other operations required to set up a valid connection, the access latency time for a UE in idle state is longer than the latency time for a UE in connected state where a radio connection has already been established between the RAN and the UTRAN.
One way to achieve lower latency is to have already-connected UEs stay in that RRC connected state for as long as possible, even if there is little or no data to be sent currently over the connection. But the problem with this approach is that it results in a greater control signaling load that must be communicated over the radio air interface and processed in the RAN. In connected state, it is the RAN's responsibility to keep track of the mobile and maintain connections as the mobile moves. That responsibility is called mobility management. For purposes here, the control processing (CP) load relates to the data processing load that must be performed by the base station data processing resources for mobility management and for signaling mobility management information. Non-limiting example data processing resources include data processing time, memory, power, interface bandwidth, and signaling processing for access signaling.
The control processing (CP) load includes receiving and evaluating UE mobility measurements that are conveyed by the UE to the RAN including downlink signal strength measurements of broadcast signals received by the UE from neighboring base stations in the UE's active and secondary handover (HO) candidate cell sets. The control processing (CP) load may also include signal processing necessary for extracting the control signaling from the air interface such as random-access decoding and communicating with neighboring base stations and with the core network. Such communications consume processor time and interface bandwidth. In LTE, the base station will have to handle all of this control signal data processing. Every time a UE sends HO measurement reports and every time handovers occur from or to the base station, the data processing for handling those control operations consume the base station's data processing resources. For even a moderate number of UEs, the control processing load can quickly consume the available data processing resources in the base station.
It is an object to provide technology that reduces access latency but also protects the radio network from being overloaded with mobility-related control processing.